Measurement devices are used in nearly all branches of industry for measuring physical quantities, in particular quantities related to ongoing production processes. Measurement indications indicating the value of the quantity measured by the device are for example commonly used in process automation for monitoring, controlling and/or regulating a production process at a measurement site.
There is a wide range of measurement devices on the market, like for example level measurement devices for measuring a level of a product in a container, flow meters for measuring a flow of a product through a pipe, temperature measurement devices or pressure measurement devices.
In order to ensure, that theses devices fulfill certain measurement properties specified for them, in particular a specified measurement accuracy, and/or comply to certain standards, they are re-calibrated regularly.
Calibration is a commonly used procedure for establishing a relation for obtaining a measurement result for a measured quantity from a measurement indication of a measurement device. Also calibration is used to check conformity of a device to a given specification. In both cases the measurement device performs at least one measurement task according to a given operating procedure, during which at least one given value of the quantity to be measured by the device is provided by a corresponding reference or standard. A typical operating procedure includes for example measurements of a minimal and a maximal value of the quantity, within a measurement range of the device. During the operation procedure, the values of the quantity provided by the reference or standard and the corresponding measurement indications of the measurement device are recorded. Based on this data the corresponding measurement errors are calculated, which in most cases are determined as the differences between the measurement indications and the corresponding values of the quantity to be measured provided by the reference or standard.
In addition a maximum permissible error between the values of the quantity provided by the standard or reference and the corresponding measurement indications of the device is provided. In case the measurement errors between the values of the quantity provided by the standard or reference and the corresponding measurement indications derived by the measurement device exceed the maximum permissible error, the device is considered not to conform. As a consequence, adjustment, repair or replacement of the measurement device is required. Required adjustments are usually performed based on the data obtained during the calibration procedure. They include for example adjustments of offset, gain and/or span of the measurement indication.
If the measurement errors do not exceed the maximum permissible error conformity of the device is declared and generally no further actions are taken.
Today measurement devices are generally calibrated periodically, after fixed calibration time intervals, e.g. intervals recommended by the manufacturer for the specific type of device. Thus the same calibration time interval is applied, regardless of whether a large measurement error, e.g. an error which is very close to exceeding the maximum permissible error, or a very small measurement error was determined during the last calibration.
In the first case, there is a high probability, that the measurement error of the device will exceed the maximum permissible error during the calibration time interval. If this is the case, the measurement error of the device may cause a potential danger to the people, the environment and the ongoing production process at the measurement site it is operated on in the time prior to the next calibration.
In the second case, there is a high probability, that the device will still be in full compliance at the end of the calibration time interval. Thus a much larger calibration time interval could have been applied without increasing the risk involved in operating the device. A larger calibration time interval between consecutive calibrations would be especially advantageous in applications, wherein calibrations are cost and time intensive, e.g. because they require a whole section of a production site to be shut down, in order to transfer the device from its measurement site to a calibration site.
Despite of this, it is general practice to apply fixed standard calibration time intervals to devices, which were found to be fully compliant.
In European Patent, EP 1 743 226 B1 a method is described, for determining an optimized service interval between consecutive maintenance services of a field device, which is adapted to the conditions prevailing at the operation site of the field device. According to this method, the time interval between consecutive services is determined based on service information, including application-specific parameters prevailing at an operating site of the field device, device-specific parameters, as well as historical data and calibration data from a previous calibrations of the device.
Thus service intervals of devices operating in a rough environment, which is e.g. subjected to extreme temperatures, temperature variations, pressures, and/or pressure variations, or exposed to abrasive and/or chemically aggressive products will be much shorter, than service intervals for devices operating under more friendly conditions.
Unfortunately, it is generally not safe to apply the same method in order to optimize calibration time intervals. The main reason for this is, that it is usually not possible to predict, when or on which timescale an initially fully compliant device will develop a relevant measurement error.